1. Field of the Invention
The present invention relates to an optical unit which is employed in a laser beam printer or the like to guide a laser beam generated from a laser diode to an object to be scanned. More particularly, the invention relates to an optical deflector which is incorporated in the optical unit for deflecting the laser beam toward the object to be scanned.
2. Description of the Related Art
In general, an optical unit incorporated in a laser beam printer or the like is designed such that a laser beam output from a laser diode is guided first to a scanner or an optical deflector and then to a photosensitive body, i.e., an object to be scanned. The photosensitive body is scanned with the laser beam at a constant speed. When guided from the laser diode to the photosensitive body, the laser beam passes through a group of focusing lenses. By these focusing lenses, the laser beam is given a cross sectional shape having desirable characteristic, and then falls on a predetermined position on the surface of the photosensitive body.
In this type of optical unit, the optical deflector includes a polygonal mirror having a plurality of nonplanar mirror surfaces. With this polygonal mirror being rotated, the optical deflector reflects the laser beam toward the photosensitive body at a nonuniform angular velocity. The polygonal mirror is coupled to the motor shaft of the optical deflector. In the polygonal mirror included in the above optical deflector, the number of mirror surfaces provided for the polygonal mirror is even, in many cases.
With respect to the polygonal mirror of the abovementioned type, it is known that the width of the line which is to be scanned with the laser beam deflected by one mirror surface, i.e., that the width of the line on the photosensitive body which is to be scanned in the main scanning direction during one scan, is very narrow. The width of the line to be scanned is in the range of several tens of .mu.m to 100 .mu.m or so. With respect to the polygonal mirror, it is also known that the diameter of the beam spot which the laser beam forms on the photosensitive body varies considerably if the distance between the mirror surface of the polygonal mirror and the shaft of the motor is varied, due to the eccentricity of the polygonal mirror or the tilting of each mirror surface. Therefore, the polygonal mirror and the shaft of the motor have to be coupled together with very high positional accuracy. For example, they have to be coupled together such that a variation in the distance between them is not more than 5 .mu.m or so. Therefore, they are coupled together by use of a method which easily provides high coupling accuracy. For example, they are coupled together by press fitting the shaft of the motor into the predetermined shaft hole of the polygonal mirror.
However, the press fitting of the shaft into the shaft hole of the polygonal mirror does not ensure a satisfactory result, since the shaft may be inserted in a slanted state or the polygonal mirror may receive the shaft eccentricly. Even if the shaft of the motor is inserted accurately at the center of the polygonal mirror, the shaft itself may be slanted with reference to the motor. In such a case, the shaft of the motor rotates in a runout condition, with the result that the mirror surfaces of the polygonal mirror tilt in synchronism with the rotation of the motor. These problems may be solved to some extent by using expensive structural components, such as air bearings, etc., but the use of such structural components naturally results in an increase in manufacturing cost.